Studies on the effects of changes in serum and perfusion fluid osmolarities on the volume flow of fluid into the cerebral ventricles of cats will be continued. In these experiments, the flow of fluid across the ependyma of the choroid plexus and ventricular lining will be altered under the influence of acetazolamide or during perfusion of the ventricular system with hypoosmolar fluids. Under these conditions, the origin of the newly formed fluid and its content of osmotically active substances will also be examined. Since the ion present in greatest concentration is sodium, and most of the sodium in spinal fluid is secreted along with newly formed fluid, the relationship between the influx of sodium and volume flow into the ventricles will be studied as the volume flow varies from complete inhibition to a several fold increase over normal. The effects of increased intraventricular pressure and spinal arachnoiditis on regional spinal cord blood flow will be investigated. In these experiments, the effects of inflammation above and on blood flow values can be compared to those obtained with increased pressure and with normal animals. An attempt will be made to explain the mechanism by which the central canal of the spinal cord can dilate when the direct communication of the ventricular system and the subarachnoid space is obstructed.